1. Field of the Invention
The preferred embodiment relates generally to an electric power supply system and method for charging an electric storage battery of a hybrid electric vehicle.
2. Description of the Prior Art
The powertrain of a hybrid electric vehicle includes multiple power sources, an internal combustion engine (ICE), principally used when highway speeds are sustained; an electric motor for charging electric storage batteries; and a traction motor for launching the vehicle, i.e., for accelerating the vehicle from a stopped condition.
Generally, when using sealed lead acid batteries, which are also called power batteries, a constant voltage charge method is the preferred means for charging battery cells. Under a constant voltage regime currents are limited by the internal resistance of the battery. When the battery is in a low state of charge and the internal resistance is low, in-rush currents can be very large and energy can be restored to the battery very quickly. As the battery becomes charged, it reaches a transition point where a sudden rise in internal resistance occurs and, under constant voltage, the battery will accept less and less current. This self-regulating effect prevents overcharging of the battery, leading to longer battery lifetimes. Such constant voltage charge regimes are conducted at an equalization voltage, which is the recommended voltage at which the batteries can be maintained at a high state of charge (SOC).
After this transition a low current equalization charge is required to return the complete capacity of the battery and to ensure that the individual cells within the battery are brought to an approximately equal charge state.
Most of the charging and discharging of the battery is done in a bulk region below the transition point.
Conventional hybrid electric vehicle powertrains on the other hand use energy batteries, such as NiMH or Li-ion batteries, where the battery charging and equalization is performed through an electric motor driven by the ICE as a battery charger. Since these batteries are composed of many individual cells connected to form high-voltage configuration, charging or discharging these batteries during normal vehicle operations can result in individual cells being at different voltages or state of charge, thereby resulting in unbalanced individual cells. Control of the balancing these individual cells through an equalization process is difficult due to the mismatch in power ratings of the electric motor-ICE combination and the battery power required to perform a precise equalization of the storage battery. Therefore, individual cells balancing in such powertrain requires sophisticated systems and methods in the equalization process.
With the advent of plug-in hybrid electric vehicle powertrains, there are two source available for charging and equalization of these batteries, first source being onboard power source (ICE) and the second source being the off board power source (plug-in charger).
The availability of two power sources, one being onboard and the other being off board, provides for opportunities for improving charging cost and better equalization of battery. There is a need in the industry for such a system and method that provides a mechanism for determining appropriately the charging and equalization of a storage battery in a hybrid electric system using both onboard the vehicle and off board the vehicle power sources. This system and method can determine customized battery power profiles in a hybrid electric powertrain system to improve battery charging and equalization for a vehicle with a plug-in charger together with the ICE.